1. Field of the Invention
This invention relates generally to measurement and data acquisition systems and, more particularly, to a measurement and data acquisition system including a real-time monitoring circuit for implementing a control loop.
2. Description of the Related Art
Scientists and engineers often use measurement systems to perform a variety of functions, including measurement of a physical phenomena or unit under test (UUT), test and analysis of physical phenomena, process monitoring and control, control of mechanical or electrical machinery, data logging, laboratory research, and analytical chemistry, to name a few examples.
A typical measurement system comprises a computer system with a measurement device or measurement hardware. The measurement device may be a computer-based instrument, a data acquisition device or board, a programmable logic device (PLD), an actuator, or other type of device for acquiring or generating data. The measurement device may be a card or board plugged into one of the I/O slots of the computer system, or a card or board plugged into a chassis, or an external device. For example, in a common measurement system configuration, the measurement hardware is coupled to the computer system through a PCI bus, PXI (PCI extensions for Instrumentation) bus, a GPIB (General-Purpose Interface Bus), a VXI (VME extensions for Instrumentation) bus, a serial port, parallel port, or Ethernet port of the computer system. Optionally, the measurement system includes signal conditioning devices which receive field signals and condition the signals to be acquired.
A measurement system may typically include transducers, sensors, or other detecting means for providing “field” electrical signals representing a process, physical phenomena, equipment being monitored or measured, etc. The field signals are provided to the measurement hardware. In addition, a measurement system may also typically include actuators for generating output signals for stimulating a unit under test.
Measurement systems, which may also be generally referred to as data acquisition systems, may include the process of converting a physical phenomenon (such as temperature or pressure) into an electrical signal and measuring the signal in order to extract information. Computer-based measurement and data acquisition (DAQ) systems and plug-in boards are used in a wide range of applications in the laboratory, in the field, and on the manufacturing plant floor, among others.
In a measurement or data acquisition process, analog signals may be received by a digitizer, which may reside in a DAQ device or instrumentation device. The analog signals may be received from a sensor, converted to digital data (possibly after being conditioned) by an analog-to-digital converter (ADC), and transmitted to a computer system for storage and/or analysis. When a measurement system generates an output analog signal, the computer system may generate digital signals that are provided to one or more digital-to-analog converters (DACs) in the DAQ device. The DACs may convert the digital signal to an output analog signal that is used, e.g., to stimulate a UUT.
Computer-based measurement and data acquisition systems may be used to implement control loop operations. A typical data acquisition system has one or more analog inputs, analog outputs, digital inputs, digital outputs and other subsystems. Each one of these subsystems may be designed to work on its own and to provide feedback to software running the system about performance and possible error conditions. A control loop application typically involves several of these subsystems working together.
Control loop operations solved with a computer system may impose a real-time requirement on the computer hardware and software, and because of that have a maximum reaction latency requirement. In other words, all the operations on the loop may need to take place within a specific period of time. Therefore, the software running on the computer typically needs to have feedback from the hardware as to whether or not this latency requirement is being meet. In control loop applications, not only may software need to know whether each resource is operating correctly and within the limits of the specific timing, but the software may additionally need to know whether all the resources meet the requirements of the control loop.
In some prior art systems, control loop applications use the feedback of each individual element of the system to try to build an overall status of the control loop and to determine whether real-time operation is being maintained. One of the drawbacks to this model is that it can become very complex. The more subsystems that are involved in the application, the more status information that may need to be kept track of by the system, which may greatly complicate the software. Second, since each subsystem needs to be monitored, this adds to the processing time of the control loop, resulting in relatively slow control loop rates. Also, in many instances all subsystems involved are required to be capable of monitoring real-time operation. Besides the fact that it may not be possible, this requirement may add to the complexity and cost of the system.